Materials and Design 30 (2009) 3318–3324

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Materials and Design

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Short Communication

Mechanical properties and apatite-forming ability of PMMA bone cements

D. Rentería-Zamarrón *, D.A. Cortés-Hernández, L. Bretado-Aragón, W. Ortega-LaraCINVESTAV IPN-Unidad Saltillo, Carr. Saltillo-Mty, Km 13, Apdo. Postal #663, C.P. 25000 Saltillo, Coahuila, Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 April 2008Accepted 29 November 2008Available online 10 December 2008

0261-3069/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.matdes.2008.11.024

* Corresponding author. Tel.: +52 844 4389600x96E-mail address: dora.cortes@cinvestav.edu.mx (D.

The in vitro bioactivity of wollastonite-containing polymethyl-methacrylate (PMMA) cements was eval-uated. Cements were prepared by varying the wollastonite content. Working times of the mixtures, aswell as the maximum temperature reached during setting, were evaluated. The bioactivity was assessedby soaking samples in a simulated body fluid (SBF) with an ionic concentration nearly equal to that ofhuman blood plasma. The compressive strength of the cements was also evaluated. Potentially bioactivePMMA cements were obtained by adding more than 20 wt% of wollastonite ceramics. The compressivestrength of the cements decreased as the wollastonite content was increased.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Polymethyl-methacrylate (PMMA) cements are widely used forthe mechanical fixation of metallic hip replacement prostheses tobone. Methyl-methacrylate monomers (MMA) are the basic com-ponent of these systems. For clinical applications, the liquidMMA is mixed with solid beads of PMMA and inserted in the hardtissue where the polymerization process is completed [1]. Poly-merizing accelerators and/or activators are also currently addedto the liquid/solid mixture. The PMMA cements were developedin the early 1960s and they have been classified as bioinert mate-rials due to the presence of fibroblastic cells at the cement–boneinterface. The PMMA allows an immediate structure support how-ever, the cement is considered to be the weak link between boneand the metallic implant, providing a barrier to direct fracturehealing. In extreme cases this may lead to loosening of the jointand the necessity of a second surgical intervention. To overcomethese problems, research has been performed in the aim to inducebioactivity and to improve the mechanical properties of organic ce-ments by adding an inorganic bioactive system [2–4]. Apart frompromoting bioactivity, the mechanical properties of PMMA ce-ments may also be improved by adding small amounts of bioactivesystems, such as hydroxyapatite (HA) [5]. Other type of cement,which formulation contains both a polymer and a ceramic hasshown to be highly bioactive [6]. This cement consists of triethyl-ene-glycol-dimethacrylate (TEGDMA) and apatite- and wollaston-ite-containing glass–ceramic (A/W) which shows also appropriatemechanical properties. Its excellent bioactivity has been attributedto the monomers content, those corresponding to TEGDMA, whichlead to a high reactivity of the A/W glass–ceramic when the ce-ment is soaked in a simulated body fluid (SBF) [6].

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80; fax: +52 844 4389610.Rentería-Zamarrón).

A PMMA/silica nanocomposite showing both high fracturetoughness and appropriate bioactivity was also developed [7]. Itshigh fracture toughness was achieved by reducing the covalentlybonded siloxane linkages.

On the other hand, the high bioactivity of wollastonite ceramicshas been demonstrated [8,9]. The reaction between wollastoniteand SBF starts with an ionic exchange of Ca2+ for 2H+. This reactiontransforms the wollastonite crystals into an amorphous silicaphase and increases the calcium concentration and the pH of thesurrounding SBF, giving the condition for apatite nucleation andsimultaneous dissolution of the amorphous silica. Furthermore,when a ceramic or polymeric material is immersed in SBF on abed of A/W glass–ceramic [10] or wollastonite ceramics [11,12] abone-like apatite layer is formed on the substrates. Both wollaston-ite ceramics and A/W glass–ceramic act as a supplier of calciumions increasing the supersaturation degree of the fluid with respectto apatite [13].

This work aims to promote bioactivity on PMMA cements byadding wollastonite ceramics. The effect of the wollastonite con-tent on the in vitro bioactivity of the cements and on the compres-sive strength has been evaluated. Additionally, duringpolymerization, setting and doughing times of the mixtures andthe maximum temperature reached were determined.

2. Materials and methods

2.1. Preparation of the wollastonite-containing PMMA cements (W/PMMA)

A solid and a liquid component were mixed for the preparationof cements. The solid component consisted of a powder mixture ofwollastonite (W, Gosa S.A.), PMMA beads with an average molecu-lar weight (Mw) of 350,000 (Alfa Aesar) and benzoyl peroxide(BPO, Sigma–Aldrich) as a polymerization initiator. The powders

Table 3Working times and maximum temperature reached during setting.

Identification Doughing time(min)

Setting time(min)

Maximumtemperature (�C)

W-97 – – 36.5W-68 9.36 15.33 46.8W-49 3.45 6.66 71W-39 3.17 6.60 80.5W-19 4.31 8.33 90.5W-0 (no wollastonite

added)7 10.33 98

Fig. 1. Effect of the wollastonite content on the mechanical properties.

D. Rentería-Zamarrón et al. / Materials and Design 30 (2009) 3318–3324 3319

were mixed for 1 min. The liquid component was prepared by mix-ing MMA monomer (Alfa Aesar) and N,N-dimethyl-p-toluidine(DMPT, Alfa Aesar) for 30 s. DMPT was added to the MMA mono-mer as a polymerization accelerator. Five different mixtures wereprepared by varying the wollastonite content with respect toPMMA (Table 1). The quantity of MMA, BPO and DMPT was keptconstant [2]. Each paste was prepared by mixing the solid compo-nent with the liquid for 3 min at a powder-to-liquid (P/L) mass ra-tio of 1:1 at room temperature.

2.2. Measurement of temperature and setting and doughing timesduring polymerization

After mixing the liquid and the solid for 3 min, each paste waspoured into a cylindrical mold (19 mm in diameter, 50 mm inheight) and the setting behavior was evaluated using a Vicat nee-dle (ASTM C 191). Setting temperature was recorded every 20 susing a digital thermometer (Taylor, model 9841). To determinethe doughing time (which defines the moment when the cementshould be applied), the paste was probed by finger wearing a latexglove every 20 s until the mixture separated cleanly from the glove(ASTM F451-99a).

2.3. Compressive strength

For the compressive strength evaluation, after mixing the solidwith the liquid for 3 min (Section 2.1), each paste was poured intoa previously designed mold with internal dimensions of 6 mm indiameter and 12 mm in height. After setting, the cements werekept in a desiccator before testing. The compressive load was ap-plied at a cross-head speed of 0.5 mm/min according to the speci-fied in the ASTM F-451-95 standard. Five specimens were testedfor each composition (Table 1) and the average and standard devi-ation were calculated.

2.4. In vitro bioactivity assessment

After mixing the solid and the liquid for 3 min (see proceduredescribed in Section 2.1) each paste was poured into a mold to ob-tain cylindrical specimens with 19 mm in diameter and 5 mm inheight. After setting, the specimens were ground on SiC papersranging from 600 to 2400 grit and finally polished with 1 and3 lm diamond paste. To assess bioactivity polished and unpolishedsamples were immersed in a simulated body fluid (SBF) with ionconcentration nearly equal to that of human blood plasma (Table2) [14]. For the preparation of SBF appropriate amounts of reagent

Table 1Compositions of wollastonite-containing PMMA cements.

Identification Powder (wt%) Liquid (wt%)

PMMA BPO W MMA DMPT

W-97 – 2.9 97.1 99.2 0.8W-68 29.1 2.9 68 99.2 0.8W-49 48.6 2.9 48.6 99.2 0.8W-39 58.3 2.9 38.8 99.2 0.8W-19 77.7 2.9 19.4 99.2 0.8

Table 2Ionic concentration of SBF and human blood plasma.

Na+ K+ Ca2+ Mg2+ Cl� HCO�3 HPO2�4 SO2�

4

SBF 142.0 5.0 2.5 1.5 147.8 4.2 1.0 0.5Human blood

plasma142.0 5.0 2.5 1.5 103.0 27.0 1.0 0.5

grade chemicals of NaCl, NaHCO3, KCl, K2HPO4 � 3H2O, MgCl2 �6H2O, CaCl2 � 2H2O, Na2SO4 and tris-hydroxymethyl aminometh-ane (CH2OH)3CNH2 were dissolved into deionized water and buf-fered to pH 7.25 at 36.5 �C with hydrochloric acid [14]. Each ofthe five sets of samples was immersed in 150 ml of SBF for 21 daysat physiological conditions of pH and temperature.

The surface of the cements before and after immersion in SBFwas analyzed by Scanning Electron Microscopy (SEM, XL-30, Phil-

Fig. 2. XRD patterns of the cement mixtures before immersion in SBF.

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lips), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction(XRD, Xpert-P3040, Philips) and Fourier-Transformed-InfraredSpectroscopy (FT-IR). In the aim to determine Ca and P ion concen-

Fig. 3. SEM and EDS analysis of the c

tration, the remaining SBF solutions were analyzed by InductivelyCoupled Plasma Atomic Emission Spectroscopy (ICP-AES, FMO-02M, Spectroflame).

ements before immersion in SBF.

Table 4Functional groups present in the spectra shown in Fig. 4.

Wavenumber (cm�1) Functional group

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3. Results and discussion

Table 3 shows the setting and doughing times of the W/PMMAcements and the maximum temperature reached during polymer-ization. When the highest content of wollastonite was added (sam-ple W-97), the doughing and setting times were over 20 min, whilethose corresponding to the wollastonite-free sample (W-O) were10.33 and 7 min, respectively. Generally, these working times de-creased as the ceramic content was increased up to 49 wt%. Thus,the handling properties of samples W-19, W-39 and W-49 are sim-ilar. As expected, the samples with more than 50 wt% of wollaston-ite took longer time for setting due to the low content of PMMAbeads in the formulation.

The maximum temperature reached during setting decreasedconsiderably as the ceramic content was increased. According tothe literature [15], the ceramic present in the organic cements ab-sorbs part of the heat produced by the exothermic polymerizationreaction. This decrease in the polymerization temperature willpotentially reduce the tissue damage when implanted.

Fig. 1 shows the compressive strength of the PMMA cements asa function of the ceramic content. As observed, the strength de-creased as the wollastonite content was increased up to 68 wt%,where a value of about 40 MPa was obtained. The sample W-97,which has the highest content of wollastonite, shows an appropri-ate strength however, its setting time is extremely long (more than20 min). According to the ASTM F-451, a minimum value of 70 MPais required for bone cements, thus only W-19 and W-39 (19 and39 wt% of wollastonite) meet this mechanical requirement.

Fig. 2 shows the XRD patterns of the cements before immersionin SBF. In all the cases, wollastonite and an amorphous phase weredetected. The amorphous phase may correspond to PMMA, sincethe sample identified as W-19, which has the highest content ofthis polymer, shows an amorphous halo of higher intensity. Thesample with no PMMA added (W-97) shows a small amorphoushalo due to the liquid content (mainly MMA).

Fig. 4. FT-IR spectra of the W/PMMA cements.

Fig. 3 shows SEM images and the corresponding EDS spectra ofthe samples before immersion in SBF. As corroborated by the EDSspectrum, which shows the presence of Ca and Si, the sample W-97consists mainly of wollastonite particles. The rest of the samplesconsist of polymer beads surrounded by wollastonite particles.The presence of C is due to the monomer added in this formulation.

Fig. 4 shows the FT-IR spectra of the W/PMMA samples beforeimmersion in SBF. The functional groups described in Table 4 arepresent in all the samples, these groups correspond to both wollas-tonite and PMMA.

Fig. 5 shows the XRD patterns of the samples identified as W-39and W-49 after 21 days of immersion in SBF. As observed, apatitewas formed on the surface of these samples. This indicates the fea-sibility to obtain potential bioactive materials by adding wollas-tonite to PMMA before mixing this polymer with the liquidphase (MMA + DMPT). No apatite was detected by XRD on the restof the samples (W-97, W-68 and W-19) after 21 days of immersionin SBF. A high bioactivity of W-97 and W-68 was expected due tothe high content of wollastonite ceramics (97 and 68 wt%, respec-tively). It seems that a higher content of PMMA is required to pro-mote polymerization and to avoid the encapsulation of the ceramicparticles in MMA, which inhibits bioactivity.

Fig. 6 shows the SEM images and corresponding EDS spectra ofthe W/PMMA specimens after 21 days of immersion in SBF. A thickand homogenous ceramic layer, constituted by small Ca, P-richagglomerates was formed on the samples W-49 and W-39. Similarresults were observed on the unpolished samples. The Ca, P-richlayer was the one identified as apatite by XRD (Fig. 5). After21 days of immersion the elements corresponding to the sub-strates (W-49 and W-39) were no longer detected by EDS. The

(1) 3000–3500 @CH–NH2

(2) 2300–3000 Methacrylamide(3) 1742 Carbonyl C@O(4) 1491 a-Methyl (CH3)(5) 1281 a-Methyl (CH3)(6) 1205 CH chain(7) 998 RHC@CH2

(8) 700–1200 Si–O(9) 576 CH3 + CH(10) 550 C–O

Fig. 5. XRD patterns of the samples after 21 days of immersion in SBF.

Fig. 6. SEM images and EDS spectra of the samples after 21 days of immersion in SBF.

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Ca/P atomic ratio of the layers formed on these samples was calcu-lated from the EDS results and this value was 1.65 for the layerformed on W-49 and 1.56 for that one formed on W-49. These val-

ues are close to the Ca/P ratio of hydroxyapatite (1.67). On the W-97 and W-68 samples a Ca, P-rich layer was also detected by SEMand EDS however, this layer was not completely covering the sub-

Fig. 7. FT-IR spectra of the samples after 21 days of immersion in SBF.

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strate, which explains the reason for not detecting this layer byXRD. Larger agglomerates were formed on the W-97 and W-68samples in comparison to those formed on W-49 and W-39. Thequantity of wollastonite added to the sample W-19 (19 wt%) wasnot enough to promote the formation of the Ca, P-rich agglomer-ates on this sample.

Fig. 7 shows the FT-IR spectra of the samples W-39 and W-49after 21 days of immersion in SBF. These spectra are similar tothose of the samples before immersion (Fig. 4 and Table 4), func-tional groups corresponding to PMMA and wollastonite were de-tected. Furthermore, in the spectra of W-39 and W-49, a newband appeared at 940–1080 cm�1, corresponding to P–O vibrationmode. The results obtained by FT-IR are in good agreement withthose of XRD, SEM and EDS analysis. No a significant change wasobserved on the rest of the samples by using FT-IR.

Fig. 8 shows the change in concentrations in Ca and P in the SBFwith the immersion time. The Ca concentration increases slightlyduring the first 7 days of immersion. According to the literature,the apatite formation is mainly due to the Ca2+ ions released intothe SBF from the wollastonite leading to the increase of the super-saturation degree of the fluid with respect to apatite by increasingpH [16]. After 7 days of immersion, the Ca concentration in SBF de-creases due to the formation of apatite on the samples and this de-crease is greater for the W-49 sample, which has a higherwollastonite content (49 wt%) than W-39 (39 wt%). This may indi-cate that the in vitro bioactivity of W-49 is higher than that of W-39. The P concentration decreases considerably in the SBF solutions

Fig. 8. Change in Ca and P concentrations in SBF as a function of soaking time.

corresponding to both of the samples due to the incorporation ofthis element in the apatitic structure.

According to the results obtained, the sample W-19 (19.4 wt% ofwollastonite + 77.7 wt% of PMMA) is not bioactive. The behaviorobserved during the characterization of the sample W-19 may bedue to the low content of wollastonite and the high content ofPMMA, being the polymer an inhibitor of bioactivity. By addingmore than 20 wt% of wollastonite, highly bioactive cements are ob-tained. However, by increasing the wollastonite content in themonomeric samples (W-97, 97 wt% of wollastonite + MMA, with-out adding PMMA beads) a slightly bioactive behavior was ob-served. This may be explained taking into account the longsetting time measured in this sample (longer than 20 min). Poly-merization of this sample may not be complete, leading to mono-mer-free wollastonite particles that promote a slight bioactivity.

4. Conclusions

Potential bioactive PMMA cements can be obtained by addingmore than 20 wt% of wollastonite ceramics. A higher bioactivitywas observed on cements formulated with 39 and 49 wt% of cera-mic (W-39 and W-49, respectively). After 21 days of immersion inSBF a homogeneous and thick apatite layer was observed on thesamples W-39 and W-49, while the Ca, P-rich agglomerates ob-served by SEM and EDS on the samples containing 68 and97 wt% of wollastonite (W-68 and W-97, respectively) were notdetected by XRD.

The handling behavior of the highly bioactive cements (W-39and W-49) is similar: the doughing time was between 3 and4 min and the setting time was about 7 min. In comparison withthe wollastonite-free cement, the doughing and setting time de-creased as the wollastonite content was increased up to 49 wt%.At higher ceramic content (68 and 97 wt%), these times increaseconsiderably.

The temperature reached by the cement mixtures during poly-merization and the compressive strength of the hard cements de-creased substantially as the wollastonite content was increased.

Among the formulations prepared in this work, the cement with39 wt% of wollastonite showed both bioactivity and appropriatecompressive strength. A further research needs to be performedby varying the powder-to-liquid ratio, however the results ob-tained in this work indicate that wollastonite can be used forobtaining bioactive PMMA bone cements.

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